In recent years, as the demand for indoor voice communication and data communication has grown due to the spread of mobile phones, the development of a home-use base station installed indoors has been pursued. As a form of operation of such a home-use base station, a way of implementing communication in which only a pre-registered mobile phone(s) is connected to a home-use base station has been studied. Since a range covered by a home-use base station is considerably smaller than that of a base station installed outdoors, the range is called “femtocell”. Accordingly, a home-use base station is referred to as “femto base station” hereinafter.
Femto base stations as well as base stations in existing mobile communication networks transmit a common pilot signal. A mobile station performs synchronization establishment, channel estimation, and the like by receiving such a common pilot signal, and then performs data transmission/reception with a base station. Therefore, it is necessary to be able to receive a common pilot signal with appropriate receiving quality in a mobile station in order to provide appropriate communication quality.
In base stations in existing mobile communication networks, the transmission power of a common pilot signal to be transmitted in each cell is set in advance to a fixed value. In contrast to this, for common pilot signals transmitted by femto base stations in femtocells, a way of autonomously setting the transmission power by a femto base station has been studied. Patent document 1 (page 14, line 8 to page 15, line 21) discloses a method like this.
Its specific example is explained with reference to FIG. 8. Referring to FIG. 8, a macro base station 81 forms a macrocell 801 and transmits a common pilot signal CP1 with a constant transmission power to communicate with a mobile station (not shown). Femto base stations 91A and 91B form femtocells 802A and 802B respectively. Further, each of the femto base stations 91A and 91B measures a received power Pmacro [dBm] of the common pilot signal CP1 of the macro base station 81, and they transmit common pilot signals CP2A and CP2B respectively with a transmission power Pmacro+Poffset [dBm] by using the same radio frequency as that of the macro base station 81 to communicate with a mobile station (not shown). Note that Poffset is a constant value common to all the femtocells 802A and 802B.
The femto base station like the one described above has been studied for use in systems such as WCDMA and E-UTRAN. In WCDMA, data transmission is performed by using an individual channel, for which transmission power is controlled, on a uplink line and a downlink line, or is performed by using a shared channel on a downlink line as shown in Non-patent document 1. Further, in E-UTRAN, a radio frequency band is divided into a plurality of PRBs (Physical Resource Blocks) as shown in Non-patent document 2. A scheduler provided in an E-UTRAN base station assigns PRBs, and a base station performs data transmission with a mobile station by using an assigned PRB.
[Patent Document 1]
UK Patent Application Publication No. 2428937 A
[Patent Document 2]
Japanese Unexamined Patent Application Publication No. 10-013909
[Patent Document 3]
Published Japanese Translation of PCT International Publication for Patent Application, No. 2005-515648
[Patent Document 4]
Japanese Unexamined Patent Application Publication No. 2005-073290
[Non Patent Document 1]
3GPP TS 25.214 V7.3.0 (2006-12), 3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Physical layer procedures (FDD) (Release 7)
[Non Patent Document 2]
3GPP TS 36.300 V8.1.0 (2007-06), 3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Evolved Universal Terrestrial Radio Access (E-UTRA) and Evolved Universal Terrestrial Radio Access Network (E-UTRAN); Overall description; Stage 2